For example, JP4569371B2 (corresponding to US2006/0243938A1) recites a linear solenoid, which includes a plunger made of a magnetic material, a yoke made of a magnetic material and a stator core. The stator core includes a magnetically attracting core, a slide core and a magnetic shield portion, which are formed integrally.
In this linear solenoid, the plunger is placed on a radially inner side of a coil and is movable in an axial direction. The yoke is configured into a cup form and includes an opening, a bottom wall portion and a peripheral wall portion. The peripheral wall portion covers an outer peripheral portion of the coil, and the bottom portion covers one axial end of the coil.
The magnetically attracting core of the stator core is made of a magnetic material and magnetically attracts the plunger toward the other axial side, which is opposite from the one axial end of the coil, with a magnetic flux generated through energization of the coil. The slide core is made of the magnetic material and is configured into a tubular form. The slide core is placed on the radially inner side of the coil and covers an outer peripheral portion of the plunger. The slide core axially slidably supports the plunger and conducts the magnetic flux between the slide core and the plunger in a radial direction. The magnetic shield portion limits flow of the magnetic flux between the magnetically attracting core and the slide core.
The stator core is inserted into an inside of the yoke from one axial side of the stator core where the slide core is located, and the stator core is fixed to the yoke at the opening of the yoke.
In the linear solenoid of JP4569371 B2 (corresponding to US2006/0243938A1), breathing passages are formed to enable smooth axial movement of the plunger at the time of starting the energization of the coil and at the time of stopping the energization of the coil.
The breathing passages include a first breathing passage, which is formed as a through-hole that axially extends through the plunger to conduct the fluid between one end side of the plunger and the other end side of the plunger. The breathing passages also include a second breathing passage, which conducts the fluid between the inside and an outside of the yoke at a location around the one axial end of the plunger and is formed as an annular gap (also referred to as an annular space) between the outer peripheral portion of the coil and the peripheral wall portion of the yoke. This annular gap is communicated to the outside of the yoke through a terminal output opening of the yoke.
However, since the second breathing passage is formed between the outer peripheral portion of the coil and the peripheral wall portion of the yoke in the linear solenoid of JP4569371B2 (corresponding to US2006/0243938A1), it is required to increase the inner diameter of the yoke to form the annular gap between the outer peripheral portion of the coil and the peripheral wall portion of the yoke. Therefore, the radial size of the linear solenoid is disadvantageously increased. As a result, in order to meet a demand of increasing the number of winding turns of the coil and a demand of reducing the radial size while forming the second breathing passage, another structure is required.
JP4569371B2 (corresponding to US2006/0243938A1) and JP2012-241733A teach a breathing groove formed in the bottom wall portion of the yoke. However, even in the case where the breathing groove is formed in the bottom wall portion of the yoke, it is still required to form the annular gap between the outer peripheral portion of the coil and the peripheral wall portion of the yoke to form the second breathing passage. Thus, it is still required to communicate the annular gap to the outside of the yoke through the terminal output opening of the yoke, and thereby the radial size is disadvantageously increased.